Development of tusks and associated structures used in aggressive intrasexual interactions in Sowerby’s Beaked Whale, Mesoplodon bidens.

This study examined the development of the tusks and associated skeletal structures in a series of 18 specimens of Sowerby's beaked whale (Mesoplodon bidens) from Scotland. The relative position of the tusks along the length of the mandible was found to move from 23% of mandible length from the anterior extremity in the smallest animal to around 37% in adults, due to a lengthening of the symphyseal region. A previously undescribed sexual dimorphism was noted, a bony abutment, or ossicular dental support, situated directly posterior to the erupted tusks of adult males. Ontogenetic changes associated with male maturity include an increase in tusk length, an increase in the length of the alveolus, a high level of wear on tusks, an increase in the depth of the mandible posterior to the alveolus, an increased level of mesorostral ossification and an overall thickening of the rostrum and the symphyseal region of the mandible. The tusks are presumed to be used in intraspecific aggressive interactions and the ossicular dental support may counteract backward and outward forces on them as they cut through an opponents flesh, while the thickening of the rostrum and mandible may reduce the risk of damage when the two animals make contact.